The present disclosure relates to aircraft environmental control systems. Air cycle machines are used in environmental control systems in aircraft to condition air for delivery to an aircraft cabin. Conditioned air is air at a temperature, pressure, and humidity desirable for aircraft passenger comfort and safety. At or near ground level, the ambient air temperature and/or humidity is often sufficiently high that the air must be cooled as part of the conditioning process before being delivered to the aircraft cabin. At flight altitude, ambient air is often far cooler than desired, but at such a low pressure that it must be compressed to an acceptable pressure as part of the conditioning process. Compressing ambient air at flight altitude heats the resulting pressured air sufficiently that it must be cooled, even if the ambient air temperature is very low. Thus, under most conditions, heat must be removed from air by the air cycle machine before the air is delivered to the aircraft cabin.
A cabin air compressor can be used to compress air for use in an environmental control system. The cabin air compressor includes a motor to drive a compressor section that in turn compresses air flowing through the cabin air compressor. The motor drives a tie rod, which causes the tie rod to rotate and drive the compressor section. An air bearing assembly keeps the tie rod aligned and supports the tie rod as the tie rod rotates. An air bearing shaft is connected to the tie rod and rotates with the tie rod. A stationary bearing sleeve surrounds the air bearing shaft and is lined with bearing foil. When the air bearing shaft rotates with the tie rod, an air film is generated between the bearing sleeve and the air bearing shaft such that the air bearing shaft does not contact the bearing foil or bearing sleeve. When the shaft slows down, the shaft makes contact with the bearing foil, and heats up due to friction between the bearing foil and the shaft. This friction can cause excessive heat buildup, which can cause significant wear of the air bearing components and also lead to premature bearing failure.